Force sensor

ABSTRACT

A force sensor, in particular for detecting the forces on a vehicle seat, includes a force measurement cell ( 1 ) that has a Hall element. The force measurement cell ( 1 ) includes at least one bending bar ( 2, 3 ), which from the exertion of force to be detected exeris an influence on the magnetic field in the region of a magnetic-field-sensitive sensor element ( 6 ) of the measurement cell ( 1 ). The sensing is done with a Hall element ( 6 ), held on the at least one bending bar ( 2, 3 ), which element, under the force exerted on the bending bar ( 2, 3 ), can be deflected in the field of a relatively stationary permanent magnet ( 4 ), and a magnetic diagnosis field in the region of the Hall element ( 6 ) can be generated whose field lines are located in the plane of the sensor element without influencing the measurement field.

This application is a 371 of PCT/DE03/01013.

BACKGROUND OF THE INVENTION

The invention relates to a force sensor, in particular for detectingforces on a vehicle seat, which by utilizing electrical orelectromagnetic effects outputs an electrical signal which correspondsto a compressive force, tensile force or bending force acting on theforce sensor.

In many fields in mechanics, force sensors are needed which even atrelatively inaccessible places in equipment or assemblies are meant toenable precise determination of the tensile and/or compressive forces.An electrical signal corresponding to the force measurement is intendedto be available for further processes of evaluation or regulation. Inthe field of automotive electronics, for instance, such force sensorscan often be used, but the force sensors used until now are mostlyrelatively large, and the production process is relatively expensive.

For some important electronic systems in the motor vehicle, such as forbelt restraint systems, a small, compact force sensor is needed.Precisely at the connecting points between components, at which pointsthe force is concentrated, force measuring bolts as a component of theforce sensor are of particular importance. For instance, for measuringthe force of the weight on a seat and its distribution, which ismeasured at the connection to the seat, a force sensor is needed whichcan be mass produced economically in large numbers. But in the fields ofproduction and quality measurement as well, precise force sensors thatmeasure statically are increasingly needed.

From International Patent Disclosure WO 00/16054 A1, a measurementpickup for detecting motion in a vehicle seat is known, in which anelastic deformation of a supporting element is detected by themeasurement cell between an upper frame, including the seating shell,and a lower frame, secured to the floor of the vehicle, optionally alsovia a mechanism for longitudinal and vertical adjustment.

It is also known per se for such a measurement cell to have a Hallelement with an integrated circuit connected to it, with which element amagnetic field that is altered because of a mechanical deformation canbe evaluated. Until now, in the known embodiment, the self-diagnosisthat is important for reliable function, such as the diagnosispertaining to security against mispolarization and the diagnosis of thesensor connection, the bond pins, and other IC-specific details, hasbeen limited to the integrated sensor circuit (sensor IC).

SUMMARY OF THE INVENTION

A force sensor of the type described at the outset, particularly fordetecting the forces on a vehicle seat, with a force measurement cellthat has a Hall element is advantageously refined in such a way that themeasurement cell includes at least one bending bar, which from the forceexertion to be detected exerts influence on the magnetic field in theregion of a magnetic-field-sensitive sensor element. The sensor elementcomprises a Hall element, retained on the at least one bending bar,which under the force exerted on the bending bar can be deflected in thefield of a relatively stationary permanent magnet. According to theinvention, a magnetic diagnosis field in the region of the Hall elementcan advantageously be generated, whose field lines are located in theplane of the sensor element without influencing the measurement field.

The diagnosis that is advantageously possible here goes beyondself-diagnosis of the integrated circuit in the Hall element withrespect to short-circuit capabilities of the pins to preventmispolarization and other short circuits. Since the entire force sensoralso comprises mechanical components, such as the bending bar and apermanent magnet, its functional monitoring during operation canadvantageously be improved with the invention.

To that end, only one additional coil is needed in the force sensor, forgenerating a diagnosis field. The excitation of the coil can be done ina simple way via an alternating voltage signal, which is modulated up tothe supply voltage. A diode in the control unit prevents the currentthrough the coil, and a capacitor can also be connected in series withthe coil in order to form a series resonant circuit. In this way, thesensor can be operated with a requisite voltage supply, and the coil cansimultaneously be operated with an alternating voltage without anydirect component.

The permanent magnet is repelled or attracted by the magnetic field ofthe coil depending on the instantaneous sign of the alternating voltageand thus on the induced field. The field direction is located in theplane of the Hall element and thus does not alter the magnetic flux inthe sensitive direction of the Hall element.

The total force sensor has two pronounced mechanical resonantfrequencies. The first results from only the mechanical dimension of theholder of the permanent magnet; the second resonant frequency isdetermined by the bending bar and is higher, but both resonantfrequencies are located outside the measurement frequency of the Hallelement.

If the coil is excited at the resonant frequency of the magnet holder,then its periodic deflection is obtained, which in turn can be measuredas an output signal of the integrated circuit of the Hall element (HallIC). In this way, according to the invention, information is obtainedabout the functioning of the entire system, without requiring anadditional connection. For instance, if the entire suspension of themeasurement cell is broken, then the stop-limit gap closes, and thebending bar can no longer oscillate. If the permanent magnet loses itsproperties or comes loose in some way, oscillatory excitation is againno longer possible.

The alternating voltage for exciting the coil can be derived in a simpleway directly from the clock signal of a microprocessor, in this case theCPU of the integrated circuit. The excitation of the coil can either bedone continuously or in only chronologically limited pulses, in whichcase the decay is utilized for diagnosis. For monitoring the resonantfrequencies of the structure, because of the very strong permanentmagnet, only little energy is needed for the coil triggering events;that is, accordingly only low voltages and currents are necessary.

In summary, it can be stated that by means of the detection of theresonant frequencies and/or the resonant amplitudes according to theinvention, additional diagnostic redundance for complete sensormonitoring is created at little expense and without additionalconnections.

BRIEF DESCRPTION OF THE DRAWINGS

One exemplary embodiment of a force sensor of the invention fordetecting the forces on a vehicle seat will be explained in conjunctionwith the drawing. Shown are:

FIG. 1, a section through a force measurement cell for a vehicle seat ina motor vehicle, having a Hall element as its sensor element; and

FIG. 2, a detail of a section through a Hall force sensor with a coilfor generating a diagnosis field.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a force measurement cell or force measuring bolt 1 for aforce sensor is shown, for instance for detecting the weight on avehicle seat, not explained in detail here, of a motor vehicle. Theforce measurement cell 1 has two bending bars 2 and 3. On a fixed end, apermanent magnet 4 is also held by means of a nonmagnetic pin 5press-fitted into a bore, while on the other end, which is moved by thebending bar, there is a Hall element 6 with a connected integratedcircuit (Hall IC); this circuit furnishes an electrical output signaldependent on the deflection of the bending bars 2 and 3.

From a detail view in FIG. 2 showing the permanent magnet 4 and the Hallelement or Hall IC 6, an additional coil 7 for generating a diagnosisfield can be seen. The excitation of the coil 7 is done with analternating voltage signal, which is modulated up to the supply voltageU+; a diode D in the control unit prevents a direct current from flowingthrough the coil 7. The field direction of the magnetic field generatedby the coil 7 is located in the plane of the Hall element 6 and thusdoes not alter the magnetic flow in the sensitive direction of the Hallelement 6.

For self-diagnosis of the arrangement, the coil 7 is for instance eitherexcited at the resonant frequency of the magnet holder 4, 5 or of thebending bars 2 and 3, resulting in their periodic deflection, which inturn can be measured as an output signal of the integrated circuit ofthe Hall element (Hall IC) 6, so that thus the complete mode ofoperation of the arrangement can be monitored.

1. A force sensor, comprising, a force measurement cell (1) that has aHall element, wherein the force measurement cell (1) includes at leastone bending bar (2, 3), which on the basis of the exertion of force tobe detected exerts an influence on the magnetic field In the region of amagnetic-field-Sensitive sensor element (6) of the farce measurementcell (1); wherein the sensor element comprises a Hall element (8),retained on the at least one bending bar (2, 3), which element can bedeflected by the force exerted on the bending bar (2, 3) within thefield of a permanent magnet (4) disposed in relatively stationaryfashion; wherein a magnetic diagnosis field in the region of the Hallelement (6) can be generated, the field lines of which are located inthe plane of the Hall element (6) without influencing themeasurementent; wherein the force measurement cell (1) is employed fordetecting the forces on a vehicle seat of a motor vehicle; and whereinthe voltage supply of a coil (7) for generating the diagnosis field inthe plane of the Hall element (6) is effected by means of an alternatingvoltage, modulated up to the supply voltage of the Hall element (6), adiode (D) or a capacitor is connected in series in a supply line for thecoil (7).
 2. The force sensor of claim 1, wherein the diagnosis field isan alternating field, whose frequency is outside the measurementfrequency of the sensor element (6) and includes at least one resonantfrequency of the mechanical makeup of the measurement cell (1).
 3. Theforce sensor of claim 2, wherein a first resonant frequency ispredetermined by the mechanical dimensioning of a holder of thepermanent magnet (4).
 4. The force sensor of claim 2, wherein a secondresonant frequency is predetermined by the mechanical dimensioning ofthe bending bar (2, 3) of the measurement cell (1).
 5. The force sensorof claim 1, wherein the coil (7) is wrapped around the Hall element (6).6. The force sensor of claim 1, wherein the upward-module alternatingvoltage is derived from the clock signal of a microprocessor of theintegrated circuit of the Hall element (6).
 7. The force sensor of claim6, wherein the alternating voltage is continuously present.
 8. The forcesensor of claim 6, wherein the alternating voltage is present inchronologically limited pulses, and the decay of the alternating voltagecan be used for diagnosis.